LTE (Long Term Evolution) cellular systems, as set forth in the LTE specifications of the 3rd Generation Partnership Project (3GPP), provide for four different quality of service (QoS) classes, referred to as traffic classes. They are conversational class, streaming class, interactive class, and background class. The main distinguishing factor between these QoS classes is how delay sensitive the traffic is. Conversational class is meant for traffic which is very delay sensitive while background class is the most delay insensitive traffic class.
In LTE systems, a mobile terminal, referred to as user equipment (UE), initially acquires uplink (UL) resources from the base station, referred to as an evolved node B (eNB) for transmitting data by performing a random access (RA) procedure using the physical random access channel (PRACH). The PRACH is a specific set of time-frequency resources allocated by eNB for use by UEs in performing the RA procedure. If periodic scheduling request (SR) resources are not allocated to the UE by the network, the random access process is also used by the UE to acquire uplink resources for all subsequent data transmission.
PRACH resources are shared among users who are in connected mode as well as users who are in idle mode. Since the same PRACH resources are used for random access procedure by multiple UEs, collisions can occur. The PRACH is expected to operate with very low collision rates, but if the number of users increases, the collision rate increases as well as the error rate. The LTE standard specifies a back-off mechanism for preamble retransmission for general users in order to avoid further collisions, but there is no solution specific to users running background traffic. As noted above, background traffic is different in the sense that it does not have a stringent latency requirement and has relatively very small packet transmissions.